Immunobiology

🛡️Immunobiology Unit 11 – Immune Tolerance and Autoimmunity

Immune tolerance is the body's ability to recognize and not attack its own cells. This delicate balance prevents autoimmunity, where the immune system mistakenly targets self-antigens. Central and peripheral tolerance mechanisms work together to maintain this equilibrium. When tolerance fails, autoimmune diseases can develop. These conditions range from organ-specific to systemic, impacting various body parts. Diagnosis involves detecting autoantibodies and assessing organ function. Treatment aims to suppress the immune response and manage symptoms.

Key Concepts

  • Immune tolerance involves the immune system's ability to recognize and not react to self-antigens, preventing autoimmunity
  • Central tolerance occurs in the thymus and bone marrow, where self-reactive lymphocytes are eliminated or inactivated
    • Mediated by processes such as clonal deletion, receptor editing, and anergy
  • Peripheral tolerance takes place in secondary lymphoid organs and tissues, regulating self-reactive lymphocytes that escape central tolerance
    • Mechanisms include suppression by regulatory T cells (Tregs), ignorance, and apoptosis
  • Autoimmunity arises when immune tolerance fails, leading to an immune response against self-antigens
  • Autoimmune diseases can be organ-specific (type 1 diabetes) or systemic (systemic lupus erythematosus)
  • Genetic predisposition, environmental factors, and immunological imbalances contribute to the development of autoimmunity
  • Diagnosis of autoimmune diseases involves detecting autoantibodies, assessing organ function, and imaging studies
  • Treatment strategies aim to suppress the immune response, manage symptoms, and prevent complications

Mechanisms of Immune Tolerance

  • Clonal deletion eliminates self-reactive T and B cells during their development in the thymus and bone marrow, respectively
    • Mediated by the presentation of self-antigens by antigen-presenting cells (APCs)
    • Cells that bind strongly to self-antigens undergo apoptosis
  • Receptor editing allows B cells to modify their antigen receptors, potentially changing their specificity away from self-antigens
  • Anergy induces a state of unresponsiveness in self-reactive lymphocytes that escape clonal deletion
    • Occurs when cells encounter self-antigens without appropriate co-stimulatory signals
  • Regulatory T cells (Tregs) suppress the activation and function of self-reactive T cells in the periphery
    • Secrete immunosuppressive cytokines (IL-10, TGF-β) and express inhibitory receptors (CTLA-4, PD-1)
  • Ignorance refers to the lack of interaction between self-reactive lymphocytes and their cognate antigens due to spatial separation or low antigen concentration
  • Apoptosis of self-reactive cells in the periphery is induced by the engagement of death receptors (Fas) or the withdrawal of survival factors

Types of Autoimmunity

  • Organ-specific autoimmunity targets specific organs or tissues, such as the pancreas in type 1 diabetes or the thyroid in Hashimoto's thyroiditis
    • Characterized by the presence of autoantibodies and T cells specific to organ-specific antigens
  • Systemic autoimmunity affects multiple organs and tissues throughout the body, as seen in systemic lupus erythematosus (SLE) and rheumatoid arthritis
    • Involves the production of autoantibodies against widely distributed antigens (nuclear components, DNA)
  • Autoimmune phenomena can also occur as a result of infections, malignancies, or immunodeficiencies
    • Molecular mimicry between pathogen and self-antigens can trigger cross-reactive immune responses
  • Autoinflammatory diseases, such as familial Mediterranean fever, involve innate immune dysregulation without autoantibodies or self-reactive T cells
  • Polyautoimmunity refers to the presence of multiple distinct autoimmune diseases in an individual

Causes and Risk Factors

  • Genetic predisposition plays a significant role in the development of autoimmunity, with multiple genes contributing to disease susceptibility
    • Human leukocyte antigen (HLA) genes are strongly associated with many autoimmune diseases
  • Environmental factors can trigger autoimmunity in genetically susceptible individuals
    • Infections, particularly viral infections, can initiate or exacerbate autoimmune responses through molecular mimicry or bystander activation
    • Toxins, drugs, and other chemical agents can modify self-antigens or alter immune regulation
  • Hormonal factors influence the development of autoimmunity, with many diseases being more prevalent in females
    • Estrogens can enhance humoral immunity and promote the survival of autoreactive cells
  • Immunological imbalances, such as defects in regulatory T cell function or overactive B cell responses, contribute to the loss of tolerance
  • Age is a risk factor, with many autoimmune diseases onset occurring in adulthood
  • Microbiome alterations and gut dysbiosis may disrupt immune homeostasis and promote autoimmunity

Autoimmune Diseases

  • Type 1 diabetes results from the autoimmune destruction of insulin-producing beta cells in the pancreas
    • Leads to insulin deficiency and hyperglycemia
  • Multiple sclerosis (MS) is characterized by autoimmune attacks on the myelin sheath of nerve fibers in the central nervous system
    • Causes neurological symptoms, including vision problems, muscle weakness, and coordination difficulties
  • Rheumatoid arthritis (RA) involves autoimmune inflammation of the synovial joints, leading to pain, swelling, and joint damage
    • Autoantibodies (rheumatoid factor, anti-citrullinated protein antibodies) are present in many cases
  • Systemic lupus erythematosus (SLE) is a systemic autoimmune disease affecting multiple organs, including the skin, joints, kidneys, and brain
    • Characterized by the production of autoantibodies against nuclear components (anti-nuclear antibodies)
  • Inflammatory bowel diseases (IBD), such as Crohn's disease and ulcerative colitis, involve autoimmune inflammation of the gastrointestinal tract
  • Psoriasis is an autoimmune skin disorder characterized by the formation of scaly, itchy patches
  • Autoimmune thyroid diseases, including Hashimoto's thyroiditis and Graves' disease, affect the function of the thyroid gland

Diagnosis and Testing

  • Autoantibody tests detect the presence of specific autoantibodies in the blood, aiding in the diagnosis of autoimmune diseases
    • Examples include anti-nuclear antibodies (ANA) in SLE, anti-citrullinated protein antibodies (ACPA) in RA, and anti-thyroid peroxidase (TPO) antibodies in Hashimoto's thyroiditis
  • Organ-specific functional tests assess the impact of autoimmunity on the affected organs
    • Glucose tolerance tests and insulin levels in type 1 diabetes
    • Thyroid function tests (TSH, T4) in autoimmune thyroid diseases
  • Imaging studies visualize the affected organs and detect inflammation or damage
    • Magnetic resonance imaging (MRI) in MS to detect brain and spinal cord lesions
    • Joint radiographs in RA to assess joint damage and erosions
  • Biopsy of affected tissues can reveal characteristic histological changes and confirm the diagnosis
    • Skin biopsy in psoriasis shows epidermal hyperplasia and inflammatory infiltrates
  • Genetic testing may identify predisposing genetic variants, particularly in familial cases or early-onset disease
  • Differential diagnosis is crucial to exclude other conditions with similar clinical presentations

Treatment Approaches

  • Immunosuppressive medications are used to dampen the overactive immune response in autoimmune diseases
    • Corticosteroids (prednisone) have broad immunosuppressive effects and are often used as initial therapy
    • Disease-modifying antirheumatic drugs (DMARDs), such as methotrexate and sulfasalazine, slow disease progression in RA and other conditions
  • Biologic therapies target specific components of the immune system involved in autoimmunity
    • Monoclonal antibodies against TNF-α (infliximab, adalimumab) are effective in RA, IBD, and psoriasis
    • B cell depletion therapy (rituximab) is used in RA and SLE
  • Organ-specific treatments address the consequences of autoimmune damage
    • Insulin replacement therapy in type 1 diabetes
    • Thyroid hormone replacement in Hashimoto's thyroiditis
  • Lifestyle modifications, such as diet and stress management, can help manage symptoms and improve overall well-being
  • Physical therapy and rehabilitation are important for maintaining function and preventing disability in conditions like MS and RA
  • Regular monitoring of disease activity and treatment response is essential to optimize outcomes and minimize side effects

Current Research and Future Directions

  • Identifying novel autoantibodies and biomarkers to improve the diagnosis and monitoring of autoimmune diseases
    • Development of antigen-specific assays for early detection and disease stratification
  • Investigating the role of the microbiome in the development and progression of autoimmunity
    • Exploring the potential of microbiome-targeted therapies (probiotics, fecal microbiota transplantation)
  • Developing antigen-specific immunotherapies to induce tolerance to specific self-antigens
    • Peptide immunotherapy in type 1 diabetes and MS
    • Chimeric antigen receptor (CAR) T cell therapy targeting autoreactive cells
  • Advancing personalized medicine approaches based on an individual's genetic and immunological profile
    • Pharmacogenomics to predict treatment response and optimize drug selection
  • Exploring the use of stem cell therapies to regenerate damaged tissues and organs
    • Autologous hematopoietic stem cell transplantation in severe autoimmune diseases
  • Investigating the role of epigenetic modifications in the development and progression of autoimmunity
    • Targeting epigenetic regulators (histone deacetylase inhibitors, DNA methylation modifiers) as potential therapies
  • Conducting large-scale, collaborative studies to unravel the complex interactions between genetic, environmental, and immunological factors in autoimmunity


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© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.